Interaction of murine oncornaviruses with lymphocytes and various conditions of the host cells which can regulate the interaction is being studied. Experiments are performed both in vitro and in isolated in vivo systems developed in the laboratory. Among various cellular conditions studied are differentiation, activation with various stimuli, and hormonal activation. We hope to delineate some putative "co-factors" of viral infection and leukemogenesis. BIBLIOGRAPHIC REFERENCES: Cerny, J., Proffitt, M.R. and Essex, M. Immunosuppression by Moloney leukemia virus: Lack of correlation between virus replication and the immunosuppressive effect. J. Nat. Cancer Inst. 56:819, 1976. Forger, J.M., III and Cerny, J. Thymic hormone modulation of leukemogenic virus replication. Cancer Res. 36:2048, 1976.